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trouble with neglected coleman... repair, replace?

I don't expect any DIY advice, even if some of this sounds like I'm asking for it. I know just enough to be dangerous and I realize that and I *will* find a contractor for this work.

Here's my deal. I'm lease/purchasing a house with an agreement that anything I have to put into it applies to equity when I finally purchase the place, so even if big dollars come into play I get them back eventually, but fronting the money is a problem.

The house has a split system, a coleman "800" series, if I read it correctly, that was supposedly installed in the early 80's. It's a single speed natural gas furnace with direct ignition, with the air conditioner's a-coil directly above.

First problem... one day I come home and notice it's rather warm, the A/C is on, and there's no air flow. I walk down to the basement where the unit is located, and there's water everywhere. Drain line's clogged and water's streaming from holes in the ducting, covering the base of the control space ont he furnace, just everywhere. I pulled the drain line off and there was a plug of rust, cleaned that out and it drained. Problem solved.... or not. The condensor freezes solid after about an hour of use, even when the humidity's not bad and the temperature differential isn't big. I figure the charge must be low, something I can't handle, and the a-coil's probably corroded beyond repair.

Second problem... I wake up one night during this dogwood winter we're having and it's 60 degrees in the house. I hear the forced draft blower running, but no blower, even set to "on". I check the furnace, reset it, and I can see the spark from the DI, but no flame. Gas valve, pressure sensor, centrifugal switch... I know how to fix just about anything but I'm not willing to kill myself with a stupid mistake.

So... and I know I'm stretching to ask this without pictures or more data (which I can get) but... what's going to be the recommendation here, repairs to one or both, replacing just the A/C, or replacing the entire system up to the ducts? And who's a good contractor to use here in Knoxville, around the east end of town?

The installation's not in a bad place, it's in the garage, all the ducting's right there, back the truck right into the garage and drop the unit off if you want... only thing I might be concerned about is that the outlets in the ducts may be a little undersized.

If you need to know more, I'm not afraid to crawl around, measure things, take photos, and research model numbers, floor space, etc. for y'all.....

EDIT: The reason I'm asking now, and why this is important, is that I have a roommate/tenant moving in *monday*. The house stays pretty stable between ~65 and 75 right now, but it's going to get warmer...

An early 80's system is over twenty years old. I wouldn't throw good money after bad. If you plan to be in this house for any length of time, a new system is the best way to go. Might hurt a little now but it'll pay off over time. Better comfort, lower operating costs, less down time.

Specifically, your freeze-up problem is likely airflow related rather than charge. If you look into cleaning the indoor coil and blower wheel on equipment this old vs. changing out the whole system, pound for pound you're ahead with new equipment.

There is a point of diminishing return in regard to keeping old stuff alive. It can be done and is done all the time, but it may not be the best option for you.

Electricity makes refrigeration happen.

Refrigeration makes the HVAC psychrometric process happen.

HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.

*nod* ok, I'm an easy push into a replacement system.... all the RUST on this one makes me suspect.

So, the question remains.... how to choose the right system (or letting the contractor choose it, which becomes a quesiton of how to choose the right contractor) and how to pay for it, since I'm up to my ears in it already and may have a whopping $300 a month left out of everything to pay for this.

Is it going to be worthwhile to stay with a mixed system or go ahead and get a fully electric setup? Both gas and electricity are fairly cheap here, but the house leaks heat like a seive due to lots of 50's vintage single pane wooden windows. I'll be fixing that a little later now, since I've got to spring for a full HVAC replacement.

Also, the compressor unit is wayyyy on the other side of the house from the inside unit. Would it be worthwhile to keep the existing refer lines and use the existing pad and wiring, or put a new pad in and have the new compressor unit approximately 10' from the inside unit on the other side of a wall? the only reason I can see for having the compressor where it is, is that it's located directly underneath the fuse panel for the house.

Is it going to be worthwhile to stay with a mixed system or go ahead and get a fully electric setup? Both gas and electricity are fairly cheap here, but the house leaks heat like a seive due to lots of 50's vintage single pane wooden windows. I'll be fixing that a little later now, since I've got to spring for a full HVAC replacement.

This poses somewhat of a dilemma...if you intend to make significant thermal improvements to your home (insulation, energy efficient windows, etc), that will change the heat loss and gain factors for your house and will affect equipment sizing. At the same time, your present system is on the fritz so you can't just go after thermal improvements while cruising with the present system and then get the sizing of a new system done once all the thermal improvements are complete.

In any case I would have a heat load calculation done for your house. Click on the bull's eye at the top of almost every web page on this site and purchase a homeowner's version of Don's great HVAC-Calc software. Well worth the small price. You will then have a reference for when contractors come out for the bidding process.

You'll have to determine if natural gas is still more cost effective than a heat pump system with auxiliary electric heat. Offhand I'd say if you went with a bare minimum of an 80% efficient gas furnace and a 10 SEER a/c, you'll be ahead of the curve over what you have now.

A new lineset is always a good idea with a new system, so if you want to move your condenser location when you get a new system, that's the best time to do it. Especially if rerouting the electrical feed and low voltage wiring isn't too difficult.

Electricity makes refrigeration happen.

Refrigeration makes the HVAC psychrometric process happen.

HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.

check to see if your pressure switch is making. Sounds like a good time to have a contractor come in and give you his opinions. water in exhaust maybe, freezing up is no air flow or low on refrigerant. That old of furnace have him water test the heat exchanger while he's at it.

If you're speaking of the forced draft pressure switch... there isn't one. All it has is this little flow vane micro switch that the vane has come off of, that's why prev(2).owner rigged it stuck closed.

Heh, this furnace... I'm reading everything I can on it, come to find that not only is there no control board (has a transformer and a relay) , the flame sensor is just a rod... no thermocouple. Simple as heck but I'm going to be happy when it goes, even if I manage to fix it.

I'm going to call KUB today to see if they have any reason why my gas would be shut off (just changed utilities to my name, you never know) and ask if they can check my outside regulator, and find a good contractor here in Knoxville.

Any leads on contractors in Knoxville, TN? The little finder up at the top of page didn't send much back but Sears...

I'm not so concerned about the A/C just yet but may have the charge tested just to make sure, and get the coil cleaned. The way it's put together I'll have to cut apart the vent's tape seals to get to it. Cobbled piece of...